A large landmass usually referred to as the mainland.
A continent is a Grid Structure consisting of more then 30 sims, placed in direct contact one to another. The geographical features continue from one sim to another, forming a multisim structure. The transportation ways (roads, railways, waterways) also expand from one sim to another, to create a continental network. Also, a continents are visible separated by Oceans or Void Ocean, but sometimes they are linked together with little channels of protected waterways - Transcontinental Channels. The only exception is Nautilus - Satori. Technically, they form a single structure, but since they were historical separated before creation of Nautilus City, they are treated as two distinct continents. A part of a continent is a Subcontinent.
If the sims are less then 30 but they are linked together, have common geographic features and intersim transportation ways, they form a Microcontinent.
Also, if the sims are not well connected together (even if there are more then 30 sims), they form a Sim Cluster. An even smaller structure is a Sim Group, while sims that cannot be grouped are Unassociated Sims. In remote places, there can be found structures known as Isolated Sim.
Currently, there are 28 continents, 18 are Linden-owned, while 10 are private-owned.
The largest continents are mainland (that means they are Linden-owned). However, a lot of smaller continents are private-owned, usually by a single estate.
List Of Continents
There are many lists, created in various periods of time. Majority of the maps show only the mainland continents, while some other autors include more or less developed private continents. Since Second Life is a dinamic world, the number of continents and the continent structures will change from time to time. The most complete list was created in August 2013 and describes 24 existing continents and a 25th, deleted. Their maps can be accessed using this download link: 
The map (click for zoom) was made in August 1st 2013. All continents that have more then 30 sims are shown. After that, 7new continents were created, while a single one vanished from the grid. The up-to-date list can be checked by the link below: